This invention relates broadly to the field of earth boring and more particularly to earth boring by a drill bit driven by a downhole electric motor supplied with electrical power from the surface.
The continuing search for supplies of oil and gas has recently resulted in the drilling of wells to greater depths than has been the previous practice. At these relative greater depths, 10,000 feet or about 2 miles and below, the standard rotary drilling rigs have previously been limited in drilling capability, since the length of the tubular drilling string severely limited the rotational speeds at which the drilling bit was rotated. This was primarily due to risk of twisting off the drilling string in the borehole and the resulting loss of time to recover the drill bit and lower portion of the string or loss of the borehole itself. Below 15,000 feet, for example, drillers have been hesitant to rotate the drilling string at more than 35 r.p.m.
At such low drill bit rotational speeds, the long life diamond drilling bits used at such depths to minimize the frequency of replacement have characteristically had low formation penetration rates as such bits usually achieve maximum economic formation penetration when rotated at speeds in the range of 400- 800 r.p.m.
Some prior art attempts to improve drilling operations using downhole motors to rotate the drill bits at high speed include U.S. Pat. Nos. 2,531,120; 2,803,433 and 3,007,534. The latter patent employs an electrical swivel for rotating a transmission power cable with the drilling string, but the cable is carried exteriorly of the drill string where it is easily damaged by contact with the borehole walls. The U.S. Pat. Nos. 2,531,120 and 3,696,332 disclose drill pipe having electrical conductors formed integral therein which automatically connect electrically when the drill pipe is made up in the drilling string, but such numerous connections increase the electrical resistancce and decrease the power communicated to the drilling motor. U.S. Pat. No. 3,280,923 attempts to avoid the problem of supplying power to the bit motor by employing a special nuclear reactor downhole to generate the electrical power for the bit motor.
Three U.S. Pat. Nos. 3,285,629; 3,372,762 and 3,777,827 disclose electrical drilling systems employing flexible hose as the drilling fluid circulating pipe instead of the standard tubular drill string and the electrical connections are made at the same time as the 500 feet long sections of flex hose are connected. Such an arrangement has not been particularly suitable for use with the standard joints of drill pipe in deep drilling, since the additional time needed in making the large number of connections required has been very costly.
U.S. Pat. No. 3,378,811 disclosed an electrical connector system by which an electrical connector may pass downwardly through the bore of a plurality of previously connected drill pipes for establishing a downhole electrical connection to avoid the necessity of using separate sections of electrical cable for each section of drill pipe. Electrical connectors for use in an oil field environment have also been disclosed in U.S. Pat. Nos. 3,657,681 and 3,753,206 while the electrical connector disclosed in U.S. Pat. No. 3,714,384 was directed for use in submerged oil and gas production systems.
Additional patent disclosures directed to making an electrical connection in a hostile environment include U.S. Pat. Nos. 3,398,392; 3,522,576; 3,596,231; 3,643,207 and 3,729,699.